Surface crosslinked polyethylene

ABSTRACT

A method for producing a wear resistant polyethylene medical implant includes forming a medical implant, such as an orthopedic implant, made at least partially of ultra high molecular weight polyethylene (UHMWPE). The polyethylene may be irradiated with gamma ray or e-beam radiation to form free radicals and then crosslinked to eliminate free radicals prior to exposure to oxygen. The so treated bearing surface of the crosslinked polyethylene is then coated with a photoinitiator. Thereafter the bearing material is photocrosslinked with ultra-violet (UV) radiation. The photocrosslinking process can also be applied to non-crosslink UHMWPE.

BACKGROUND OF THE INVENTION

Ultra High molecular weight polyethylene (UHMWPE) has been used as abearing material for artificial joints such as hips and knees. Highlycrosslinked UHMWPE is the current state-of-the-art for orthopedicbearing applications. Gamma or e-beam irradiation is the standardprocess to crosslink UHMWPE with practical irradiation dose about 3-10Mrads. In this dose range, the wear rate is about 2.5 mm3/Mc at 9.5Mrads and 20 mm3/Mc at 3.3 Mrads, respectively (See FIG. 22, U.S. Pat.No. 6,800,670 B2). Continuing increase of irradiation dose from 10 to 50Mrads gradually drops the wear rate towards zero, but the materialbecomes too brittle for clinical use. It is desirable for UHMWPE thathave wear rate about zero without increasing irradiation dose above 10Mrads, thus mechanical properties does not suffer.

This invention relates to medical implants formed of a polymericmaterial such as ultra-high molecular weight polyethylene, with superioroxidation and wear resistance produced by an irradiation and annealingprocess followed by UV surface crosslinking. Alternatively the UHMWPEbase material could be virgin UHMWPE or doped with an antioxidant suchas anthocyanin.

Various polymer systems have been used for the preparation of artificialprostheses for biomedical use, particularly orthopedic applications.Among them, ultra-high molecular weight polyethylene is widely used forarticulation surfaces in artificial knee, hip, and other jointreplacements. Ultra-high molecular weight polyethylene (UHMWPE) has beendefined as those linear polyethylenes which have a relative viscosity of2.3 or greater at a solution concentration of 0.05% at 135° C. indecahydronaphthalene. The nominal weight−average molecular weight is atleast 400,000 and up to 10,000,000 and usually from three to sixmillion. The manufacturing process begins with the polymer beingsupplied as fine powder which is consolidated into various forms, suchas rods and slabs, using ram extrusion or compression molding.Afterwards, the consolidated rods or slabs are machined into the finalshape of the orthopedic implant components. Alternatively, the componentcan be produced by compression molding of the UHMWPE resin powder.

It has been recognized that regardless of the radiation type, the highenergy beam causes generation of free radicals in polymers duringradiation. It has also been recognized that the amount or number of freeradicals generated is dependent upon the radiation dose received by thepolymers and that the distribution of free radicals in the polymericimplant depends upon the geometry of the component, the type of polymer,the dose rate, and the type of radiation beam. The generation of freeradicals can be described by the following reaction (which usespolyolefin and gamma ray irradiation for illustration):

If oxygen is present, primary free radicals r• will react with oxygenand the polymer according to the following reactions as described in“Radiation Effects on Polymers,” edited by Roger L. Clough and ShalabyW. Shalaby, published by American Chemical Society, Washington, D.C.,1991.

In the Presence of Oxygen

In radiation in air, primary free radicals r• will react with oxygen toform peroxyl free radicals r0₂., which then react with polyolefin (suchas UHMWPE) to start the oxidative chain scission reactions (reactions 2through 6). Through these reactions, material properties of the plastic,such as molecular weight, tensile and wear properties, are degraded.

It has been found that the hydroperoxides (rOOH and POOH) formed inreactions 3 and 5 will slowly break down as shown in reaction 7 toinitiate post-radiation degradation. Reactions and 9 representtermination steps of free radicals to form ester or carbon-carboncross-links. Depending on the type of polymer, the extent of reactions 8and 9 in relation to reactions 2 through 7 may vary. For irradiatedUHMWPE, a value of 0.3 for the ratio of chain scission to cross-linkinghas been obtained, indicating that even though cross-linking is adominant mechanism, a significant amount of chain scission occurs inirradiated polyethylene.

By applying radiation in an inert atmosphere, since there is no oxidantpresent, the primary free radicals r• or secondary free radicals P• canonly react with other neighboring free radicals to form carbon-carboncross-links, according to reactions 10 through 12 below. If all the freeradicals react through reactions 10 through 12, there will be no chainscission and there will be no molecular weight degradation. Furthermore,the extent of cross-linking is increased over the original polymer priorto irradiation. On the other hand, if not all the free radicals formedare combined through reactions 10, 11 and 12, then some free radicalswill remain in the plastic component.

Out of Contact with Oxygen

It is recognized that the fewer the free radicals, the better thepolymer retains its physical properties over time. The greater thenumber of free radicals, the greater the degree of molecular weight andpolymer property degradation will occur. Applicant has discovered thatthe extent of completion of free radical cross-linking reactions isdependent on the reaction rates and the time period given for reactionto occur.

UHMWPE is commonly used to make prosthetic joints such as artificial hipjoints. In recent years, it has been found that tissue necrosis andinterface osteolysis may occur in response to UHMWPE wear debris. Forexample, wear of acetabular cups of UHMWPE in artificial hip joints mayintroduce microscopic wear particles into the surrounding tissues.

Improving the wear resistance of the UHMWPE socket and, thereby,reducing the rate of production of wear debris may extend the usefullife of artificial joints and permit them to be used successfully inyounger patients. Consequently, numerous modifications in physicalproperties of UHMWPE have been proposed to improve its wear resistance.

It is known in the art that ultrahigh molecular weight polyethylene(UHMWPE) can be cross-linked by irradiation with high energy radiation,for example gamma or e-beam radiation, in an inert atmosphere or vacuum.Exposure of UHMWPE to gamma irradiation induces a number of free-radicalreactions in the polymer. One of these is cross-linking. Thiscross-linking creates a 3-dimensional network in the polymer whichrenders it more resistant to adhesive wear in multiple directions. Thefree radicals formed upon irradiation of UHMWPE can also participate inoxidation which reduces the molecular weight of the polymer via chainscission, leading to degradation of physical properties, embrittlementand a significant increase in wear rate. The free radicals are verylong-lived (greater than eight years), so that oxidation continues overa very long period of time resulting in an increase in the wear rate asa result of oxidation over the life of the implant.

Sun et al. U.S. Pat. No. 5,414,049, the teachings of which areincorporated herein by reference, broadly discloses the use of radiationto form free radicals and heat to form cross-links between the freeradicals prior to oxidation.

Hyun et al. U.S. Pat. No. 6,168,626 relates to a process for formingoriented UHMWPE materials for use in artificial joints by irradiatingwith low doses of high-energy radiation in an inert gas or vacuum tocross-link the material to a low degree, heating the irradiated materialto a temperature at which compressive deformation is possible,preferably to a temperature near the melting point or higher, andperforming compressive deformation followed by cooling and solidifyingthe material. The oriented UHMWPE materials have improved wearresistance. Medical implants may be machined from the oriented materialsor molded directly during the compressive deformation step. Theanisotropic nature of the oriented materials may render them susceptibleto deformation after machining into implants.

Salovey et al. U.S. Pat. No. 6,228,900, the teachings of which areincorporated by reference, relates to a method for enhancing thewear-resistance of polymers, including UHMWPE, by cross-linking them viairradiation in the melt.

Saum et al. U.S. Pat. No. 6,316,158 relates to a process for treatingUHMWPE using irradiation followed by thermally treating the polyethyleneat a temperature greater than 150° C. to recombine cross-links andeliminate free radicals.

Sequential crosslinking is described in U.S. Pat. No. 7,517,919, thedisclosure of which is incorporated by reference. An UHMWPE crosslinkedthree times as disclosed in U.S. Pat. No. 7,517,919 is designated hereinas “X3”, a registered trademark of Stryker Corporation.

In the present invention ultraviolet (UV) radiation is applied tophotocrosslink a UHMWPE bearing surface to generate an additionalsurface crosslinking layer on the already gamma or e-beam crosslinkedbulk UHMWPE implant. The implant may be previously crosslinked at adosage range between 1-10 Mrads or even higher. The surface layerthickness is controlled to a depth of about 100 micrometers. In thisrange, the surface layer can last at least 5 million cycles in a wearsimulator test without showing measurable wear which can be seen bymaintaining the original machining marks on the bearing surface.

Ultraviolet (UV) light crosslinking has been used for crosslinkingpolyethylene since the 1950's. In the past UV was used to crosslinkpolyethylene (PE) bulk material, fibers, and films by mixing aphoto-initiator and PE resin, then consolidating the resin andcrosslinking under UV irradiation (see for example U.S. Pat. No.6,281,264 B1, Chen Y. L. et al., “Photocrosslinking of Polyethylene”,Journal Polymer Science, Polymer Chemistry Edition, 1989, Qu B. J., etal., “Photoinitiating Characteristics Of Benzophenone Derivatives As NewInitiators In The Photocrosslinking Of Polyethylene, Polymer Engineeringand Science, July 2001). On the other hand, PE degradation in air whenexposed in sun light (low UV intensity) is a well-known phenomena. Toprevent this, PE, an anti-UV additive is sometimes added to thepolyethylene resin. However, none of the prior art disclosed using UV asa surface crosslinking method for a consolidated UHMWPE bearing, norusing a UV method to generate an additional crosslinking layer on agamma or e-beam irradiated crosslinked UHMWPE bearing material fororthopedic applications such as in acetabular cup, glenoid bearings,tibial bearing surfaces, finger and elbow UHMWPE bearings. U.S. Pat. No.6,165,220 relates to e-beam surface crosslinking. U.S. PatentPublication No. 20070270970 relates to polymeric bearings for use in thespine.

UV crosslinking of polyethylene is a reaction of carbon center freeradicals that are generated by UV radiation. Mechanism of the UVcrosslinking of polyethylene is briefly described below. When UV lightradiates polyethylene that contains benzophenone, a photoinitiator,benzophenone absorbs UV energy and jumps to the excited triplet state.The benzophenone in the triplet excited state abstracts hydrogen frompolyethylene to generate polyethylene carbon radicals. The formed PEcarbon center radicals undergo free radical reaction to formcrosslinking.

The quantum yield of the excited triplet state is very high forbenzophenone and the triplet state is highly effective in hydrogenextraction. It is noted that there is no carbon-carbon bond breakage inthe UV crosslinking process and the carbon radicals are generated solelyby a carbon-hydrogen bond cleavage. Therefore, unlike gamma or e-beamcrosslinking process, UV crosslinking does not result in a reduction ofmolecular weight.

Due to improved surface wear performance of surface photocrosslinkedalready bulk crosslinked UHMWPE, acetabular UHMWPE cups can be designedto be very thin (<3 mm) without the danger of wearing through. Thecurrent FDA standard is 6.0 mm thick UHMWPE. A thinner cup will maximizefemoral head size. The bigger femoral head size can significantlydecrease rate of dislocation, which is the number one cause of total hipreplacement revision. Another benefit of the very thin UHMWPE bearing isto allow cobalt-chrome-on-UHMWPE for resurfacing hip applications.Current resurfacing bearing are typically cobalt chrome on cobalt chromewhich may release metal ions which causes metal hypersensitivity in somepatients.

Another benefit of the present invention is for knee tibial insertapplications. One of the causes of knee revision is that the tibialimplant is set on a soft bone bed. Overtime after implantation, the softtibia bone allows the implant to sink to a lower position and the gapbetween femoral and tibia is increased, resulting in higher impact forceaccelerated wear of tibia insert. This invention allows the tibia insertbecomes thinner, such as 3 mm, as compared to the normal 6 mm standard.The thinner tibia insert requires less of a bone cut. The bone in bothfemoral and tibia sides has a hardness gradient. The closer to thebearing portion of the knee joint, the harder the bone. Thus the thinnerbone cut allows the knee implants to be set on the stronger bone.

Typically wear rates are higher than 2.5 mm/million cycles for 10 Mradsgamma irradiated bulk UHMWPE.

Thus additional UV surface photo-crosslinking performed on gamma ray ore-beam already crosslinked material unexpectedly produces lower wearthan current crosslinked UHMWPE. The final wear performance issignificantly improved as compared to the non-UV surface crosslinkedsurface made of highly crosslinked UHMWPE. The improved photocrosslinksurface allows the use of alternate bearing material such as PAEK ingeneral and PEEK in particular.

BRIEF SUMMARY OF THE INVENTION

UV cross-linking is a non-ionized crosslinking method. During thecrosslinking process, only C—H bonds in UHMWPE are cleavaged and thencross-linked. Therefore, there is no C—C bond breaking and no molecularweight decrease possible after crosslinking.

UV only crosslinks a surface layer to improve wear resistance, while thewhole bulk materials can be kept in virgin condition or low cross-linkedcondition (such as the N2-Vac process). N2-Vac is treating the UHMWPEwith 3 Mrad in less than 1% oxygen (nitrogen) without subsequentannealing. Therefore, the whole component has high fracture toughness(FIG. 4). This is especially beneficial for tibial inserts, the UVcross-linked layer thickness could be <200 micron which is much lessthan 1-2 mm, the depth of von Mises stress under bearing surface.Therefore, the UV cross-linked bearing has low adhesion surface wear andless fatigue wear (pitting and delamination).

A method for producing a wear resistant polyethylene medical implant,such as an orthopedic implant, comprises forming a medical implant, atleast partially from ultra high molecular weight polyethylene (UHMWPE).The UHMWPE may be irradiated with gamma ray or e-beam radiation to formfree radicals followed by crosslinking the formed free radicals usingheat prior to exposure to air. Alternatively, the UHMWPE may be infusedwith an anti-oxidant (vitamin E, arothocynanin as discussed in U.S.Patent Publication No. 20100036491, the disclosure of which isincorporated herein by reference). The bearing surface of thecrosslinked polyethylene is coated with a photoinitiator. After thecoating has dried the coated surface is crosslinked with ultra-violet(UV) radiation. Preferably the photoinitiator is Benzophenone. Thecoating with the Benzophenone may be performed by vapor deposition.Alternately the coating with Benzophenone may be done by immersion in anacetone solution of Benzophenone at a preferred concentration of 1-100mg Benzophenone/ml of acetone and more preferably being 10 mg/ml. Ifcrosslinked prior to UV photocrosslinking the irradiation with gamma rayor e-beam radiation and crosslinking may be performed sequentially asdescribed in U.S. Pat. No. 7,517,919. Each radiation dose using thismethod is less than or equal to 3.0 MRads. The coating process is suchthat the photoinitiator can penetrate the UHMWPE to a depth of between200 microns and 1 mm. The depth depends on soak time in thephotoinitiator. The implant is allowed to cool between gamma ray ore-beam crosslinking and the photocrosslinking. The photocrosslinkingpreferably occurs about 65° C. or between 65° C. and room temperature.

Photoinitiators which could be used include 2-methylanthraquinone,2-ethylanthraquinone, 2-chloroanthraquinone, p-chloranil, benzylsulfide, benzyl sulfoxide, phenyl sulfoxide, 4-acetylbiphenyl, anthrone,hexachlorobenzene, benzophenone, 4,4′-dimethoxybezophenone,4-Nitrobenzophenone, 2-chlorobenzophenone, 4-chlorobenzophenone and4,4′-dichlorobenzophenone.

The UV light used in the photocrosslinking preferably has a wavelengthof 300 to 400 nm, and most preferably 350 nm. The intensity of the lightcan be up to 100 mW/cm² but preferably 50 mW/cm².

The UV cross-linking can be done in an oxygen-free medium. Theoxygen-free reaction medium includes oxygen-free water and insertatmosphere. The reaction temperature can be up to the meltingtemperature of UHMWPE (about 135° C.).

The UV cross-linking method allows selectively crosslinks only thebearing surface, while non-bearing surfaces may be masked and are notfurther surface crosslinked. This maximizes wear resistance capabilityand avoids any property degredation which may lead to breaking thecomponent (such as a polyethylene post of a tibial insert).

UV cross-linking is a low cost process, as compared to gamma and e-beamprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the gravimetric wear of cobalt chrome on asequentially crosslinked polyethylene (x3) and the same sequentiallycrosslinked polyethylene including photocrosslinking (PXL);

FIG. 2 shows a pair of comparative photographs of the photocrosslinkedcup of FIG. 1 prior to wear testing on the left and after wear testingof 2.5 million cycles on the right as obtained using aninterferometricprofilometer;

FIG. 3 shows the gravimetric wear of a PEEK surface on aphotocrosslinked ultra high molecular weight polyethylene previouslycrosslinked in an inner atmosphere by gamma irradiation and a similarcup not photocrosslinked after one million cycles;

FIG. 4 shows an Izod impact energy before and after photocrosslinking ofvirgin ultra high molecular weight polyethylene showing no fracturetoughness dropped after photocrosslinking with UV radiation;

FIG. 5 is a graph showing the volumetric wear of a cobalt chromeprosthetic femoral head on an N₂VAC treated (3 Mrad gamma ray dose in anatmosphere of <1% oxygen and no subsequent annealing) UHMWPE bearing;

FIG. 6 is a graph showing the volumetric wear of a PEEK (polyether etherKetone) prosthetic femoral head on an N₂VAC treated UHMWPE bearingcompared to the same bearing with UV photocrosslinking; and

FIG. 7 is a graph showing the volumetric wear of Virgin Antioxidantdoped UHMWPE compared to the same antioxidant treated UHMWPE with UVsurface crosslinking at 3.5 million cycles (Mc).

DETAILED DESCRIPTION Example 1

Commercially available polyethylene acetabular cups (N2Vac according tothe process of Stryker U.S. Pat. No. 5,414,049), or Virgin GUR 1020UHMWPE which was sequentially crosslinked three times per Stryker U.S.Pat. No. 7,517,919 and obtained from Stryker orthopaedics) were used.The UV light with a wavelength of 350 nm was used as a light source.Benzophenone from Aldrich was used as a photoinitiator.

The UV crosslinking process is described below. Several already bulkcrosslinked UHMWPE acetabular cups were immersed in an acetone solutionof benzophenone. After one minute, each cup was taken out from thesolution and dried under vacuum for an hour. The cups were immersed indeionized water in a glass jar that was purged with an inert gas(nitrogen) to produced oxygen free water. The glass jar was sealed andplaced in a water bath at 65° C. Each cup was then irradiated by the UVlight for up to 2 hours. After photoirradiation (which producesphotocrosslinking (PXL)), each cup was washed with acetone and water. Novisual changes were observed during this process. The photocrosslinked(PXL) cups were tested in a hip stimulator machine for its wearperformance. Cobalt chrome and plastic ball heads (PEEK) were usedagainst one of the UV crosslinked cups. Untreated cups were used ascontrol. Wear testing was conducted in house following the ASTM standardmethod. Gravimetric wear was obtained using ASTM F2025. The soak timewas 1 minute in 10 mg benzophenone per ml of acetone. The UV radiationwas 50 milliwatts per cm² and wavelength of 350 nm (average).

FIG. 1 shows gravimetric wear (mg) of a cobalt chrome femoral head on asequentially crosslinked (CoCr/X3®) cup and a similar cup withphotocrosslinking and CoCr/X3®-PXL after 2.5 million cycles. No wearfrom CoCr/X3®-PXL was detected. The UHMWPE was initially crosslinkedthree times by the process described in U.S. Pat. No. 7,517,919. Asshown in FIG. 2 it was also found that machine marks on thephotocrosslinked cup were still visible after 2.5 million cycles.

Similar results were observed in the wear testing of PEEK head on N2Vactreated cup and PEEK head on N2Vac plus PXL (FIG. 3). The cup was 28 mmin diameter with PEEK heads in a standard hip simulator. No wear wasdetected in PEEK/N2Vac-PXL after 1.0 million cycles while anon-photocrosslinked N2Vac cup produced more than 10 mg of gravimetricwear.

FIG. 4 shows an Izod impact energy before and after photocrosslinking ofvirgin ultra high molecular weight polyethylene showing no fracturetoughness dropped after photocrosslinking with UV radiation.

FIG. 5 is a graph showing the ASTM F-648-04 as tested by the volumetricwear of a cobalt chrome prosthetic femoral head on an N₂VAC treated (3Mrad gamma ray dose in an atmosphere of <1% oxygen and no subsequentannealing) UHMWPE bearing.

FIG. 6 is a graph showing the volumetric wear of a PEEK (polyether etherketone) prosthetic femoral head on an N₂VAC treated UHMWPE bearing.

FIG. 7 is a graph showing the volumetric wear of Virgin Antioxidantdoped (with anthocyanin) UHMWPE compared to the same antioxidant treatedUHMWPE with UV surface crosslinking both at 3.5 million cycles with thewear rate is in mm³/million cycles.

An alternate method of coating the UHMWPE with benzophenone utilizesPhysical Vapor Deposition process (PVD). The photoinitiator isevaporated into a gas by heating over its boiling temperature, thencondensed onto UHMWPE bearing surface to form a uniform thin film. UVlight is applied on the photoinitiator coated bearing surface andcrosslinks the surface. Non-bearing surfaces may be masked to preventbeing coated with a photoinitiator. Also a CVD method may be used touniformly coat sensitizer (photoinitiator) on UHMWPE bearing surface. Analternative bearing combination could be UV crosslinked UHMWPE on UVcrosslinked UHMWPE.

Any residual photoinitiator may advantageously be removed from preformedUHMWPE after photocrosslinking prior to fabrication of the implant.Alternately, if the implant is photocrosslinked after fabrication, theresidual photoinitiator may be removed.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for producing a wear resistant ultra high molecular weightpolyethylene medical implant comprising: forming a medical implant madeat least partially of ultra high molecular weight polyethylene;irradiating the polyethylene with gamma ray or e-beam radiation to formfree radicals and crosslinking the formed free radicals prior toexposure to air; coating a bearing surface of the crosslinkedpolyethylene with a photoinitiator; and thereafter photocrosslinking thecoated surface with ultra-violet (UV) radiation.
 2. The method as setforth in claim 1 wherein the photoinitiator is Benzophenone.
 3. Themethod as set forth in claim 2 wherein the coating with the Benzophenoneis performed by vapor deposition.
 4. The method as set forth in claim 2wherein the coating with Benzophenone is done by immersion in an acetonesolution of Benzophenone.
 5. The method as set forth in claim 1 whereinthe medical implant is an orthopedic implant.
 6. The method as set forthin claim 1 wherein the irradiation with gamma ray or e-beam radiationand crosslinking are performed in at least two sequential radiationdoses.
 7. The method as set forth in claim 6 wherein each radiation doseis less than 3.0 MRads.
 8. The method as set forth in claim 1 whereinthe UV and photoinitiator penetrate to a depth of less than about 1 mm.9. The method as set forth in claim 1 wherein the implant is allowed tocool between gamma ray or e-beam crosslinking and the photocrosslinking.10. A method of crosslinking an ultra high molecular weight polyethylene(UHMWPE) preformed medial implant comprising: irradiating the preformedUHMWPE implant with gamma ray or e-beam radiation followed by heating ina low oxygen atmosphere to form crosslinks in the UHMWPE; coating abearing surface of the preformed crosslink implant with aphotoinitiator; photocrosslinking the photoinitiator coated surface ofthe crosslinked UHMWPE with ultra violet (UV) radiation.
 11. The methodas set forth in claim 10 wherein the photoinitiator is Benzophenone. 12.The method as set forth in claim 11 wherein the coating with theBenzophenone is performed by vapor deposition.
 13. The method as setforth in claim 11 wherein the coating with Benzophenone is done byimmersion in an acetone solution of Benzophenone.
 14. The method as setforth in claim 10 wherein the medical implant is an orthopedic implant.15. The method as set forth in claim 14 wherein the medical implant isan acetabular cup or tibial insert.
 16. The method as set forth in claim10 wherein the irradiation with gamma ray or e-beam radiation andcrosslinking is performed in at least two sequential radiation doses.17. The method as set forth in claim 16 wherein each radiation dose isless than 3.0 MRads.
 18. The method as set forth in claim 10 wherein theUV and photoinitiator penetrate to a depth of less than about 200microns.
 19. An orthopedic joint comprising a first polymeric bearing; asecond polymeric bearing for articulation on the first bearing, thesecond polymeric bearing having a crosslinked bearing surface, thesurface crosslinked to a depth of less than 1 mm.
 20. The orthopedicjoint as set forth in claim 19 wherein the first polymeric bearingcomprises poly aryl ether ketone (PAEK).
 21. The orthopedic joint as setforth in claim 20 wherein the PAEK is polyether ether ketone (PEEK). 22.The orthopedic joint as set forth in claim 20 wherein the secondpolymeric bearing is Ultrahigh Molecular weight polyethylene (UHMWPE).23. The orthopedic joint as set forth in claim 22 wherein the secondpolymeric bearing is doped with an antioxidant prior to crosslinking.24. A method for producing a wear resistant Ultra High Molecular Weightpolyethylene (HHMWPE) medical implant comprising: doping UHMWPE with ananti-oxidant; coating a bearing surface of an implant made of the dopedUHMWPE with a photoinitiator; and thereafter crosslinking the bearingsurface with ultraviolet (UV) radiation.
 25. The method as set forth inclaim 24 wherein the photoinitiator penetrates less than 1 mm into thesurface of the doped UHMWPE.
 26. The method as set forth in claim 25wherein the doped UHMWPE is soaked in a mixture of Benzophenone andacetone.
 27. The method as set forth in claim 26 wherein theBenzophenone penetrated the doped UHMWPE to a depth of between 200microns and 1 mm.